In the past, most brushless DC motors used dedicated sensors such as Hall effect devices or optical sensors to determine the angular position of the rotor with respect to the stator. This information was then used to commutate the motor. More recently, the back EMF signals from the stator coils of the motor have been used to sense the angular position of the rotor with the advantages of reducing commutation timing errors and costs. However, when the motor is stationary there is no back EMF from the stator coils and so a special start system is required. This system is also required at low speeds as the drive currents cause voltages that can be much greater than the back EMF signals making their detection a problem.
Design Engineering October 1984, page 37 discloses a back EMF position sensing system for brushless DC motors. This article is not clear as to the details of the starting system used but describes a technique which only allows energisation of the motor coils during particular time periods which is incompatible with many applications.
EP-A-251785 discloses a method of producing feedback information concerning the rotational position of the rotor of a brushless dc motor without using sensing devices in the structure of the motor, when the motor is at a standstill or rotating only slowly. The phases of the motor are energised under the control of a microprocessor and current through the phases is monitored by the microprocessor. Periodically, from standstill to a first rotational speed, each of the set of motor phases is momentarily energised in sequence and the amplitude of the short current pulse that flows in each phase is monitored. The phase in which the highest current pulse flows indicates the position of the rotor. The momentary energisation of the set of phases is insufficient to generate sufficient torque to turn the rotor: each momentary energisation of the set of phases is followed by sustained torque-producing energisation of the particular phase which is appropriate to the rotor position indicated by the monitored current pulses which flowed in response to the momentary energisation. Monitoring the amplitudes of the short current pulses requires the use of an analogue-to-digital converter. Accordingly, the method disclosed in EP-A-251785 has the disadvantage of either restricting the choice of the microprocessor to one with an integral analogue-to-digital converter or requiring the use of a separate analogue-to-digital converter which would be accompanied by extra cost, power consumption and lower system reliability.
DE-C2-32 09 394 discloses a system of controlling a brushless DC motor in order to get the motor turning in the desired direction as soon as possible. With the motor at a standstill, a current pulse is applied to one of the windings and then the back EMFs in the windings are examined to determine whether the motor has begun to turn in the desired direction. If it has, the system runs the motor in a self-commutating mode. If the motor has begun to turn in the undesired direction, the system causes the windings to be energised so that the motor changes its direction of rotation. The system disclosed in this prior document is suitable where a load is not applied until the motor has reached its operating speed but is not applicable to an arrangement where the motor has a high starting load, such as in a record disk file drive.
The invention seeks to provide an improved method of starting a brushless direct current motor, particularly one having a high starting load such as one driving a record disk file.